The present disclosure relates generally to semiconductor fabrication, and more particularly, to the measurement of overlay offset of a plurality of layers of a semiconductor substrate.
Semiconductor devices are fabricated by creating a sequence of patterned and un-patterned layers where the features on patterned layers are spatially related to one another. Thus during fabrication, each patterned layer must be aligned with a previous patterned layer, and as such, the overlay between a first layer and a second layer must be taken into account. The overlay is the relative position between two or more layers of a semiconductor substrate such as, for example, a wafer. As semiconductor processes evolve to provide for smaller critical dimensions, and devices reduce in size and increase in complexity including number of layers, the alignment precision between layers becomes increasingly more important to the quality, reliability, and yield of the devices. The alignment precision is measured as overlay offset, or the distance and direction a layer is offset from precise alignment with a previous layer. Misalignment of layers can cause performance issues and even potentially causing a device to fail due to, for example, a short caused by a misaligned interconnect layer. Therefore, it is necessary to measure the overlay offset between layers during processing to allow for possible correction.
Alignment and alignment measurement techniques are known in the art such as, for example, the use of box-in-box alignment targets. However, many of these techniques are unable to perform to the accuracy which may be required for state-of-the-art processing. Other techniques such as, for example, scatterometry may have increased accuracy, but require substantial, complicated modeling, making the techniques cumbersome and time-consuming. Many techniques also require sizeable targets that occupy substantial space on the wafer and as such may take away potentially valuable wafer space. Overlay measurement techniques known in the art also may be disadvantageous as they may include sensitivity to the uniformity of the layers present on the substrate.
Accordingly, it would be desirable to provide for improved overlay offset measurement.